SFMU fuel pressure too high

chris712vt · 08-03-2008, 10:33 AM

Alright guys, I'm stumped on my "unadjustable" fuel pressure right now. Let me explain my setup, and what I've tried so far.

I think most of my mods are up to date in my profile if you need to reference that, but my fuel system consists of (95 fuel system) a Walbro 255 high-pressure, stock supply lines all the way to the rail, 450cc injectors, 97 fuel rail (eliminated stock fpr) with -4AN outlet, -6AN return line to SFMU, then SFMU's return reduces down to a 1/4" hose and back to the stock return line on the firewall.

I bought the SFMU from a 2GNT member who previously used with with 450s and 16psi, pretty much my setup, so I didn't change anything on it. When I first started the car it didn't idle well, but smoothed out after a minute and a little revving. I checked the fuel pressure and it was 42-45ish psi, with the vacuum line connected.

I tried loosening the fuel pressure screw on top, in fact so loose the screw came out, and the fuel pressure didn't change at all (but it did increase a little when I tightened it all the way down). I unplugged the vacuum line and it didn't change. I put in the little in-line vacuum reducer thing and that didn't make a difference. I tried keeping the bleed valve all the way closed, and fully open, neither made a difference. The fuel pressure just stayed at 40 some psi every time.

I decided, well 40+psi idling under vacuum or static pressure with 450s just means I'm running rich as hell, but it won't hurt the car. So I took it for a quick test drive down the street and back. It pulls hard and feels great once I hit some boost, but I need to figure out how to drop that fuel pressure down. Also, I've noticed when I first start it up after it's been off for a while it sounds like it's running on 3 cylinders, but once it warms up, or I rev it a bit, or I hit a little boost, it goes back to being normal and strong - I assume that's an effect of running so rich and possibly dousing the plugs with fuel on initial startup.

Anyway, here are a couple pictures and two videos (one shows startup and fuel pressure gauge, the other shows the test drive). Anyone have any advice or ideas as to why I can't get the fuel pressure below 40psi? I'd like it to be more like 20.

1slow97 · 08-03-2008, 11:42 AM

When i had a SFMU, My fuel pressure was around 15psi... The only way, I could get mine so low was to run a 3/8 fuel return line from the SFMU to the tank..

Locke · 08-03-2008, 05:26 PM

Your 1/4" return may be a restriction.

chris712vt · 08-03-2008, 06:45 PM

I hope that's it. I guess 3/8" hose does have a little more than twice the cross sectional area of 1/4" hose... you think that's enough to allow me to drop the fuel pressure down to the 15-20psi range? Would 1/2" be overkill? I'm not sure what else could be the problem so I'll probably try this.

1slow97 · 08-03-2008, 08:25 PM

1/2 will be overkill..... One way to check is to run a 3/8 line from the bottom of the SFMU to a gas can. Start it up and see how low, you can get you fuel pressure down.. Keep a eye on the gas can, It will fill up fast.

The best thing, I found is to use Brakeline from the SFMU to the tank...

cs82685 · 08-04-2008, 08:52 AM

I'd also say it's your return line. Run a -6an return all the way back to the tank and you should be fine

chris712vt · 08-04-2008, 09:30 AM

haha I'd do the -6AN SS all the way if it weren't so expensive. It's not under pressure, and as long as it can be protected under the car I see no problem with standard fuel hose. What does 6AN translate to as far as regular fuel hose goes anyway, around 3/8? I'm pretty sure hose will be like 1/5th the cost of an SS line. Thanks for the help guys, I'll get on this tonight or tomorrow.

Also, one thing I forgot to mention - where the SFMU has three screw ports, boost vacuum and bleed, mine is missing a screw. I have the bleed valve in, and I have a screw in the vacuum hole, but nothing in the boost hole. Do I need to track down another screw? What are those boost and vacuum screw ports used for?

cs82685 · 08-04-2008, 02:01 PM

if money's an issue, I'd go with the brakeline as suggested. I personally wouldn't be too comfortable just having a rubber hose running underneath my car unprotected against the elements

rstare · 08-05-2008, 01:38 PM

[quote=chris712vt;151571426]\ What does 6AN translate to as far as regular fuel hose goes anyway, around 3/8? \QUOTE]
-4 = 1/4"
-5 = 5/16"
-6 = 3/8"
-8 = 1/2"
-10 = 5/8"
-12 = 3/4"
-16 = 1"
For reference

rstare · 08-05-2008, 01:50 PM

Quote:

Originally Posted by chris712vt

Also, one thing I forgot to mention - where the SFMU has three screw ports, boost vacuum and bleed, mine is missing a screw. I have the bleed valve in, and I have a screw in the vacuum hole, but nothing in the boost hole. Do I need to track down another screw? What are those boost and vacuum screw ports used for?

Does anyone have any answers for this? Im having the same question on what screws and valves should be in what ports.

cs82685 · 08-05-2008, 01:55 PM

Quote:

Originally Posted by rstare

-4 = 1/4"
-5 = 5/16"
-6 = 3/8"
-8 = 1/2"
-10 = 5/8"
-12 = 3/4"
-16 = 1"
For reference

Interesting, so the number is really just however many 16ths of an inch. 5/8" = 10/16" = -10an. Now I feel retarded for not knowing that even though it makes perfect sense.

Quote:

Originally Posted by rstare

Does anyone have any answers for this? Im having the same question on what screws and valves should be in what ports.

If I have time, I'll try to snap a pic or take a look at my SFMU today or tomorrow. My bad for never getting back to you rstare, I read your PM at work and completely forgot about it when I was at home and could have done it for you. I'm sure you can find the answer on 2gnt because I know I've come across it there before.

rstare · 08-05-2008, 02:02 PM

Quote:

Originally Posted by cs82685

Interesting, so the number is really just however many 16ths of an inch. 5/8" = 10/16" = -10an. Now I feel retarded for not knowing that even though it makes perfect sense.

If I have time, I'll try to snap a pic or take a look at my SFMU today or tomorrow. My bad for never getting back to you rstare, I read your PM at work and completely forgot about it when I was at home and could have done it for you. I'm sure you can find the answer on 2gnt because I know I've come across it there before.

Lol, I actually never realized the 16ths of a inch thing either, until you pointed that out, lol. O well, I will look on 2gnt, if you could snap a picture that would be great though.
Thanks

chris712vt · 08-09-2008, 02:28 PM

Well with the 1/4" return the lowest I could get my fuel pressure was about 42psi.

Now I've finished running my new 3/8" return line from the SFMU to the tank and I can go as low as about 14psi.

Currently have it set at 18psi, we'll see how this goes. 18psi with 450ccs, Walbro 255HP, and about 15psi of boost. (I've heard some say to set the idle at around 16psi, elsewhere I've heard 20psi... I went in the middle of the two. Idles great, nice an smooth, no almost dying when I first start it. Picks up at lower RPM and pulls in boost better than before when it was super rich.

Locke · 08-09-2008, 05:10 PM

Beautiful.

rstare · 08-10-2008, 12:42 PM

Thats awesome Chris, so how did you end up setting the ports for the sfmu?

chris712vt · 08-10-2008, 03:20 PM

Haha I didn't mess with them. Screw in the vacuum port, bleed valve closed, nothing in the boost port. Seems to be functioning just fine...?

rstare · 08-10-2008, 03:49 PM

Guess thats what I will do then